Apparatus for starting engine mounted on-vehicle

ABSTRACT

In a starter for starting an on-vehicle engine, a solenoid is provided to push a pinion gear. The solenoid has an electromagnetic coil composed of a single coil and electrically separated from a motor circuit, a fixed core, and a plunger. Supply of excitation current to the electromagnetic coil allows the fixed core to be magnetized to attract the plunger. Hence, a movement of the plunger results in a push of the movable member toward the ring gear. A switch is provided in the circuit and has a contact, a movable core, and a switch coil functioning as an electromagnet attracting the movable core in response to supply of current to the switch coil. A movement of the movable core results in on/off switching operations of the switch. The switch is allowed to operate independently of the solenoid when both the switch and solenoid are controlled.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation of U.S. application Ser. No. 12/762,537 filedApr. 19, 2010, which claims priority to Japanese Patent Application Nos.2009-102214 filed Apr. 20, 2009, 2009-281589 filed Dec. 11, 2009,2010-009832 filed Jan. 20, 2010, and 2010-092197 filed Apr. 13, 2010.The disclosure of the prior application is hereby incorporated byreference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

2. Related Art

In vehicles with engines (i.e., internal combustion engines), a starteris usually used to start the engines. Though a variety of types ofstarters are known, one type of such starters is provided with a magnetfield type of electric motor having an armature and a field coil, asolenoid, and a switch. The solenoid is used to push, using a shiftlever, a pinion gear toward a ring gear attached to an on-vehicleengine. The switch turns on/off a main contact arranged in an electriccircuit for driving the motor (known as a motor circuit), in which themotor circuit drives the motor by supplying current from a battery tothe motor. This kind of starter is disclosed by Japanese Utility ModelNo. 56-42437.

In this configuration, the solenoid and the switch can be operatedindependently of each other. For example, only the solenoid is drivenfirst to make the pinion gear to engage with the ring gear, and then theswitch is operated to close the main contact so that the current issupplied to the motor. By this sequential operation technique, the motorcan be driven to start the engine after completion of engagement betweenthe pinion gear and the ring gear.

In the foregoing starter, the solenoid to push the pinion gear has anelectromagnetic coil composed of two coils. These two coils are anattraction coil to generate a magnetic force necessary for attractingthe plunger and a retention coil to generate a magnetic force necessaryfor retaining the attracted plunger. It is usually required that bothone end of the attraction coil and one end of the retention coil beelectrically connected to a connector or other electric terminalmembers. Further, the other end of the attraction coil is electricallyconnected to fixed contacts of the main contact, so that when the maincontact in the motor circuit is closed by the electric switch, theattraction coil is short-circuited via the main contact, that is, nocurrent passes through the attraction coil.

Furthermore, in the starter disclosed above, the electromagnetic coil ofthe solenoid and the filed coil of the motor are electrically connectedby a wiring member with each other. This electrical connection intendsto allow current to flow to the field coil via the electromagnetic coilwithout closing the main contact whenever the pinion gear is broughtinto contact with the ring gear axially pushed by the solenoid. In otherwords, the current flows through the field coil via the electromagneticcoil. This current flow makes the armature of the motor rotatesslightly, thus making the pinion gear rotates slightly in response totransmission of the slight rotation of the motor armature to the piniongear, thus allowing the pinion gear and the ring gear meshes on eachother.

However, in the structure disclosed by the foregoing starter, theelectric circuitry is complicated, resulting in a larger number of partsnecessary for the electric circuit. In addition, various working stepsare required for manufacturing the starter. Such working steps include astep in which one end of the attraction coil and one end of theretention coil are electrically connected to, for example, a connector,a step in which the other end of the attraction coil is electricallyconnected to the fixed contacts of the main contact, and a step in whichthe electromagnetic coil of the solenoid to push the pinion gear and thefield coil of the motor are mutually electrically connected by aconductive wire. These many working steps result in an increase in themanufacturing costs of the starter.

Additionally, the foregoing disclosed starter has a difficulty thatpermanent magnets cannot be used as the magnetic field system of themotor. That is, this starter is obliged to employ a field coil as itsmagnetic field system. The disclosed technique by the foregoingpublication cannot be applied to permanent magnet field type of motorswhich use permanent magnets in their magnetic field system.

SUMMARY OF THE INVENTION

The present invention has been made in consideration of the foregoingcircumstances, and it is an object of the present invention to provide astarter which has the solenoid to push the pinion gear and the switch toswitch open/close the main contact of the motor circuit, wherein thesolenoid and the switch can be controlled independently of each other,manufacturing steps can be reduced in number by simplifying the electriccircuitry, and permanent magnets and a field coil can be adoptedselectively by the magnetic field system of a motor.

In order to achieve the object, the present invention provides, as oneaspect thereof, a starter for a vehicle having an engine with a ringgear, comprising: an electric motor that generates a torque in responseto reception of electric power supplied from a battery via an electriccircuit electrically connecting the battery and the motor, the circuitrelaying the power; an output shaft that rotates in response toreception of the torque from the motor, the output shaft having alongitudinal direction defined as an axial direction; a movable memberhaving a pinion gear that transmits the torque to the ring gear andbeing movable on the output shaft together with the pinion gear in theaxial direction; a solenoid having an electromagnetic coil composed of asingle coil and electrically separated from the circuit, a fixed core,and a plunger, supply of excitation current to the electromagnetic coilallowing the fixed core to be magnetized to attract the plunger so thata movement of the plunger results in a push of the movable member towardthe ring gear in the axial direction; and a switch which is provided inthe circuit and which has a contact, a movable core, and a switch coilfunctioning as an electromagnet attracting the movable core in responseto supply of current to the switch coil, a movement of the movable coreresulting in on/off switching operations of the switch, the switch beingallowed to operate independently of the solenoid when both the switchand solenoid are controlled.

As described, the solenoid pushing the pinion gear has a singleelectromagnetic coil electrically separated from the circuit for themotor. Hence, the electric circuitry can be simplified compared to theconventional. In addition, the foregoing working steps which have beennecessary for manufacturing electromagnetic coils with two coils(consisting of an attraction coil and a retention coil) becomeunnecessary.

The starter according to the present invention can adopt any of apermanent magnet and a field coil as its motor field system. Even whenadopting the field coil, it is not required to introduce a step ofeclectically connecting the field coil and the electromagnetic coil ofthe pinion-pushing solenoid. Hence, a simplified electric circuitryleads to a reduction in the number of electric parts. The number ofmanufacturing steps can be reduced, which results in startermanufacturing with saved costs.

As a second aspect, the present invention provides an apparatus forstarting an engine mounted in a vehicle, comprising: a starter; anexcitation circuit through which the excitation current flows from anon-vehicle battery to the electromagnetic coil; a starter relay thatconnects the battery and the excitation circuit; a diode having acathode and an anode, the cathode being electrically connected to apositive potential side point of the electromagnetic coil and the anodebeing electrically connected to the ground; and a controller thatcontrols excitation and non-excitation operations of the electromagneticcoil via the starter relay.

In this engine starting apparatus, in response to a drive signal formthe controller, the starter relay is closed (turned on), an excitationcurrent flows from the battery to the electromagnetic coil of thepinion-pushing solenoid via the starter relay. When the controller thencommands the current to stop, the starter relay is opened (turned off),thereby cutting off the excitation current. This will cause a counterelectromotive force (i.e., a surge voltage) across the electromagneticcoil due to its inductance.

However, the diode is connected in parallel to the electromagnetic coilwith its cathode connected to the positive potential side of theelectromagnetic coil and its anode connected to the ground. Hence, thecounter electromotive force can be absorbed well by the diode, wherebyno current flows though the starter relay on account of the counterelectromotive force. No arc discharge occurs between the contacts of thestarter relay, reducing wearing of the contacts, leading to a longerduration of life of the starter.

Preferably, the apparatus is mounted in an idle stop apparatus which iscapable of automatically controlling a stop and a restart of the engine,wherein the idle stop apparatus restarts the engine during a period oftime a time instant at which the engine starts to stop to a time instantat which the engine stops completely, the engine rotating during theperiod of time due to inertia of the engine rotation.

In this preferred example, since the operations of both the solenoidpushing the pinion gear and the switch for current supply to the motorcan be controlled independently of each other, it is possible to restartthe engine during its rotation due to its inertia after an engine stopis instructed by an idle stop apparatus. In this situation, the switchcan be activated before the activation of the solenoid, so that themotor starts rotating prior to a movement of the pinion gear to the ringgear of the engine. This means that the pinion gear meshes with the ringgear in a state where a relative difference between the rotation numbersof the ring gear rotating due to inertia and that of the pinion gear isreduced. Hence, the mesh between both the gears becomes reliable.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a cross-sectional view illustrating a starter according to afirst embodiment of the present invention;

FIG. 2 is a cross-sectional view illustrating a solenoid unit (apinion-pushing solenoid and a motor electrification switch) according tothe first embodiment;

FIG. 3 is an electrical circuit diagram illustrating an apparatus forstarting an engine according to the first embodiment;

FIG. 4 is an electrical circuit diagram illustrating an apparatus forstarting an engine according to a second embodiment of the presentinvention;

FIG. 5 is a cross-sectional view illustrating a solenoid unit accordingto a third embodiment of the present invention;

FIG. 6 is a cross-sectional view illustrating an electromagnetic switchused for a starter according to conventional art;

FIG. 7 is a graph illustrating spring characteristics and attractionforce characteristics of an electromagnetic switch used for a starteraccording to conventional art and a pinion-pushing solenoid of thepresent invention; and

FIG. 8 is a cross-sectional view illustrating a solenoid unit accordingto a fourth embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

With reference to the accompanying drawings, hereinafter will bedescribed some embodiments of the present invention.

First Embodiment

Referring to FIGS. 1 to 3, an apparatus for starting an on-vehicleengine according to a first embodiment of the present invention will bedescribed.

The apparatus for starting the engine of the first embodiment includes astarter 1 that starts an on-vehicle engine EG. FIG. 1 is across-sectional view illustrating the starter 1. In the firstembodiment, the apparatus for starting the engine EG is loaded in avehicle having an idle stop system. For example, the idle stop system isable to automatically stop the engine EG when the vehicle is in pause atan intersection by a stop signal or in pause due to traffic jam or thelike.

As shown in FIG. 1, the starter 1 includes a motor 2, an output shaft 3,a shift lever 4, a pinion movable body (described later), pinion-pushingsolenoid 5, a battery (see FIG. 3) and a motor electrification switch 7.In the present embodiment, the output shaft has a longitudinaldirection, so that directions along the longitudinal direction can bedefined as an axial direction AX, directions radiating from the axialdirection AX along a plane perpendicular to the axial direction AX canbe defined as a radial direction RA, and directions around the axialdirection AX can be defined as a circumferential direction CR.

The motor 2 generates torque. The output shaft 3 is rotated beingtransmitted with the torque of the motor 2. The pinion movable body isprovided so as to be axially movable (leftward and rightward in FIG. 1)along the output shaft 3. The pinion-pushing solenoid 5 has a functionof pushing the pinion movable body in a direction opposite to the motor(leftward in FIG. 1) via the shift lever 4. The motor electrificationswitch 7 opens/closes a main contact (described later) provided at amotor circuit which supplies current from the battery 6 to the motor 2.

FIG. 3 is an electrical circuit diagram illustrating the apparatus forstarting the engine EG according to the first embodiment. For example,as shown in FIG. 3, the motor 2 is a commutator motor that includes afield magnet 8, a rectifier 9, an armature 10 and a brush 11. The fieldmagnet 8 is configured by a plurality of permanent magnets. The armature10 has an armature shaft with its one being provided with the rectifier9. The brush 11 is provided on the outer periphery of the rectifier 9.The field magnet 8 of the motor 2, which is made up of the permanentmagnets, may be replaced by a field electromagnet made up of a fieldcoil.

The output shaft 3 is disposed being aligned with the armature shaft viaa reduction gear (not shown). The torque of the motor 2 is transmittedbeing reduced by the reduction gear.

The reduction gear is a known planetary reduction gear, for example, inwhich a planetary carrier that picks up the orbital motion of aplanetary gear is provided being integrated with the output shaft 3.

The pinion movable body is configured by a clutch 12 and a pinion gear13, which will be described later.

The clutch 12 includes a spline sleeve 12 a (see FIG. 1), an outer, aninner, a roller and a roller spring. The spline sleeve 12 a ishelical-spline-fitted to the outer periphery of the output shaft 3. Theouter is provided being integrated with the spline sleeve 12 a. Theinner is relatively rotatably arranged at the inner periphery of theouter. The roller is located between the outer and the inner toconnect/disconnect torque therebetween. The roller spring has a role ofpressing the roller. The clutch 12 is provided as a one-way clutch thatunidirectionally transmits torque from the outer to the inner via theroller.

The pinion gear 13 is integrated with the inner of the clutch 12 andrelatively rotatably supported by the outer periphery of the outputshaft 3 via bearings (not shown).

The pinion-pushing solenoid 5 and the motor electrification switch 7have a solenoid coil (i.e., an electromagnetic coil) 14 and a switchcoil 15, respectively, each of which forms an electromagnet when currentis passed. A fixed core 16 is arranged between the solenoid coil 14 andthe switch coil 15 so as to be commonly used by these coils. Meanwhile,a solenoid case 17 and a switch case 18 are continuously formed in theaxial direction AX. Specifically, the solenoid case 17 and the switchcase 18 are integrally formed to provide a single overall case. In otherwords, as shown in FIG. 1, the pinion-pushing solenoid 5 and the motorelectrification switch 7 are arranged in series in the axial directionAX to integrally configure a solenoid unit and are fixed to a starterhousing 19 so as to be parallel to the motor 2. The solenoid case 17also serves as a magnetic yoke of the pinion-pushing solenoid 5, whilethe switch case 18 also serves as a magnetic yoke of the motorelectrification switch 7.

FIG. 2 is a cross-sectional view illustrating the solenoid unit (thepinion-pushing solenoid 5 and the motor electrification switch 7). Asshown in FIG. 2, the overall case has a bottomed cylindrical shape withone axial end (first end E1) (left side in FIG. 1) being provided withan annular bottom and the other axial end (second end E2) being opened.The outer diameter of the overall case is made even from the first endE1 to the second end E2. However, the solenoid case 17 forming a part ofthe overall case at the first end E1 side is ensured to be thicker thanthe switch case 18 forming a part of the overall case at the second endE2 side. In other words, the inner peripheral surface of the overallcase has a step between the solenoid case 17 and the switch case 18.

The fixed core 16 is configured being divided into an annular core plate16 a and a core portion 16 b caulked along the inner periphery of thecore plate 16 a for fixation. The core plate 16 a has an outercircumferential surface on the coil side (first end E1 side) in thethickness-wise direction, which surface is brought into contact with thestep provided at the inner periphery of the overall case, to therebyconstrain the position of the fixed core 16 on the coil side.

Referring to FIGS. 2 and 3, hereinafter are described the configurationsof the pinion-pushing solenoid 5 and the motor electrification switch 7,excepting the overall case (the solenoid case 17 and the switch case 18)and the fixed core 16.

a) The pinion-pushing solenoid 5 includes the solenoid coil 14, aplunger 20 and a joint 21. The solenoid coil 14 is arranged along theinner periphery of the solenoid case 17 that forms a part of the overallcase on the first end E1 side. The plunger 20 is made of iron anddisposed being opposed to the core portion 16 b of the fixed core 16 andis permitted to be axially movable along the inner periphery of thesolenoid coil 14. The joint 21 transmits the movement of the plunger 20to the shift lever 4.

The solenoid coil 14 is made up of a single coil and has an end which isconnected to an external connector terminal 22 (see FIG. 3) and theother end which, for example, is connected and fixed to a surface of thecore plate 16 a by welding or the like, for grounding. The externalconnector terminal 22 is connected to an electrical wiring 44 so thatexcitation current can be passed from the battery 6 to the solenoid coil14 via a starter relay 23 (see FIG. 3).

The solenoid coil 14 has an inner periphery at which a cylindricalsleeve 24 is disposed to slidably hold the outer periphery of theplunger 20.

When the fixed core 16 is magnetized with the supply of current to thesolenoid coil 14, the plunger 20 is attracted to one end face of thecore portion 16 b against the reaction force of a return spring 25disposed between the core portion 16 b and the plunger 20. Then, whenthe current supply to the solenoid coil 14 is stopped, the plunger 20 ispushed back by the reaction force of the return spring 25 in thedirection opposite to the core portion 16 b (leftward in FIG. 2).

The plunger 20 has substantially a cylindrical shape with a cylindricalhole being formed at its radially central portion. The cylindrical holeis open at one axial end of the plunger 20 and bottomed at the other endthereof.

The joint 21 having a shape of a rod is inserted into the cylindricalhole of the plunger 20 together with a drive spring 26. Thus, the joint21 has an end portion projected from the cylindrical hole of the plunger20. This end portion of the joint 21 is formed with an engagement groove21 a with which one end portion of the shift lever 4 engages. The otherend portion of the joint 21 is provided with a flange portion 21 b. Theflange portion 21 b has an outer diameter that enables the flangeportion 21 b to be slidably movable along the inner periphery of thecylindrical hole. The flange portion 21 b, being loaded by the drivespring 26, is being pressed against the bottom face of the cylindricalhole.

With the movement of the plunger 20, an end face of the pinion gear 13pushed in the direction opposite to the motor via the shift lever 4comes into contact with an end face of a ring gear 27 (see FIG. 1) whichis attached to a crank shaft of the engine EG. Then, the drive spring 26is contracted while the plunger 20 is permitted to move and attracted tothe one end surface of the core portion 16 b. Thus, the drive spring 26accumulates reaction force that allows the pinion gear 13 to engage thering gear 27.

b) The motor electrification switch 7 includes the switch coil 15, amovable core 28, a contact cover 29, two terminal bolts 30 and 31, apair of fixed contacts 32, and a movable contact 33. The switch coil 15is arranged along the inner periphery of the switch case 18 forming apart of the overall case on the second end E2 side. The movable core 28is opposed to the core portion 16 b of the fixed core 16 and ispermitted to be movable in the axial direction AX. The contact cover 29,which is made of resin, is assembled, blocking the open end, i.e. thesecond end E2, of the overall case (the open end of the switch case 18).The two terminal bolts 30 and 31 are fixed to the contact cover 29. Thepair of fixed contacts 32 are fixed to the two terminal bolts 30 and 31.The movable contact 33 electrically connects/disconnects between thepair of fixed contacts 32.

The switch coil 15 is made up of a single coil and has one end which isconnected to an external connector terminal 34 (see FIG. 3), and theother end which, for example, is connected and fixed to a surface of thecore plate 16 a by welding or the like, for grounding.

The external connector terminal 34 is connected to an electrical wiring45 so that excitation current can be passed from the battery 6 to theswitch coil 15 via a motor relay 35 (see FIG. 3). The external connectorterminals 22 and 34 are each formed, for example, of a metal plateterminal. Ends of the respective plate terminals are provided beingexternally projected in the axial direction AX from the contact cover29.

The switch coil 15 has a radially outer peripheral side on which anaxial magnetic path member 36 is arranged to form a part of a magneticpath. Also, the switch coil 15 has an axial side opposite to the fixedcore, on which a radial magnetic path member 37 is arranged to form apart of the magnetic path.

The axial magnetic path member 36 has a cylindrical shape and isinserted into the switch case 18 along the inner periphery thereof withsubstantially no gap being provided therebetween. An end face of theaxial magnetic path member 36 on the first end E1 side is brought intocontact with the outer peripheral surface of the core plate 16 a todetermine the axial position of the member 36.

The radial magnetic path member 37 is arranged perpendicular to theaxial direction AX. The radial magnetic path member 37 has a radiallyouter end surface on the first end E1 side, which surface is broughtinto contact with an axial end face of the axial magnetic path member 36to constrain the position of the member 37 with respect to the switchcoil 15. The radial magnetic path member 37 has a round opening at itsradially central portion so that the movable core 28 can movetherethrough in the axial direction AX.

The fixed core 16 is magnetized upon supply of current to the switchcoil 15. Then, the movable core 28 is attracted to the other end face ofthe core portion 16 against the reaction force of the return spring 38disposed between the core portion 16 b and the movable core 28. When thecurrent supply to the switch coil 15 is stopped, the movable core 28 ispushed back in the direction opposite to the core portion (rightward inFIG. 2) by the reaction force of the return spring 38.

The contact cover 29 has a cylindrical trunk portion 29 a. The trunkportion 29 a is inserted into the switch case 18 along the innerperiphery thereof, the switch case 18 forming a part of the overall caseon the second end E2 side. The contact cover 29 is arranged, with theaxial end face of the trunk portion 29 a being in contact with a surfaceof the radial magnetic path member 37, and caulked and fixed to the openend, i.e. the second end E2, of the overall case.

The terminal bolt 30, one of the two terminal bolts, is connected to abattery cable 39 (see FIG. 3). The terminal bolt 31, the other of thetwo terminal bolts, is connected to a motor lead 40 (see FIGS. 1 and 3).This motor lead 40 serves as an electric circuit connecting the battery6 and the motor 2 (that is, serves as a motor circuit).

The pair of fixed contacts 32, which are provided separately from (ormay be provided integrally with) the two terminal bolts 30 and 31, areelectrically fixed to the two terminal bolts 30 and 31 inside thecontact cover 29.

The movable contact 33 is arranged so that the distance from the movablecontact 33 to the movable core is larger than the distance from the pairof fixed contacts 32 to the movable core (rightward in FIG. 2). Themovable contact 33 is in reception of the load of a contact spring 42and pressed against an end face of a resin rod 41 fixed to the movablecore 28. It should be appreciated that the initial load of the returnspring 38 is set larger than that of the contact spring 42. Therefore,when the switch coil 15 is de-energized, the movable contact 33 isseated on an inner seat 29 b (see FIG. 2) of the contact cover 29, withthe contact spring 42 being contracted.

The main contact is formed of the pair of fixed contacts 32 and themovable contact 33. Being biased by the contact spring 42, the movablecontact 33 comes into contact with the pair of fixed contacts 32 with agood pressing force. Resultantly, current is passed across the pair offixed contacts 32 to thereby close (turn on) the main contact. When themovable contact 33 is drawn apart from the pair of fixed contacts 32,the current across the pair of fixed contacts 32 is shut down to therebyopen (turn off) the main contact.

The operation of the starter 1 will be described.

The operation of the starter 1 is controlled by an ECU (electroniccontrol unit) 43 through the starter relay 23 and the motor relay 35.

a) The case where the engine EG is normally started (i.e. the case wherethe user turns on an ignition switch (not shown) to start the engine EGin the state where the engine EG is fully stopped)

When an engine start signal issued by a turn-on operation of theignition switch is inputted, the ECU 43 outputs a drive signal (turn-onsignal) to the starter relay 23. Then, the starter relay 23 is turned onso that current is passed from the battery 6 to the solenoid coil 14 ofthe pinion-pushing solenoid 5, for magnetization of the core portion 16b. Then, the plunger 20 is permitted to move being attracted to themagnetized core portion 16 b. With the movement of the plunger 20, thepinion movable body (the clutch 12 and the pinion gear 13) is pushed inthe direction opposite to the motor via the shift lever 4. Then, an endface of the pinion gear 13 comes into contact with an end face of thering gear 27 and stops.

After expiration of a predetermined period (e.g., 30 to 40 ms) from theissuance of the engine start signal, the ECU 43 outputs a drive signal(turn-on signal) to the motor relay 35 to turn on the motor relay 35.Thus, current is passed from the battery 6 to the switch coil 15 of themotor electrification switch 7 to allow the movable core 28 to beattracted to the core portion 16 b. Then, the movable contact 33 isbrought into contact with the pair of fixed contacts 32 and biased bythe contact spring 42 to thereby close the main contact. As a result,current is supplied to the motor 2 to generate torque in the armature10. The torque is then transmitted to the output shaft 3 via thereduction gear. The torque of the output shaft 3 is further transmittedto the pinion gear 13 via the clutch 12. When the pinion gear 13 rotatesup to a position that enables engagement with the ring gear 27, thepinion gear 13 is permitted to engage the ring gear 27 by the reactionforce accumulated in the drive spring 26. Thus, the torque istransmitted from the pinion gear 13 to the ring gear 27, whereby theengine EG is started.

b) The case where engine restart is requested in an engine stop processperformed by an idle stop system, and where the engine EG is restartedduring inert revolutions prior to the full stop of the engine EG.

When conditions for automatically stopping the engine EG (e.g. thevehicle speed being zero, the brake pedal being stepped on, and thelike) from an idling state are met, the ECU 43 outputs an engine stopsignal to stop fuel injection and supply of intake air. As a result, theengine EG enters an engine stop process, whereby the ring gear 27 startsdecreasing revolutions. When engine restart is requested while the ringgear 27 is decreasing revolutions (prior to the full stop of the enginerevolutions), the ECU 43 outputs a drive signal (turn-on signal) to themotor relay 35. Upon output of the drive signal, the motor relay 35 isturned so that current is passed from the battery 6 to the switch coil15. As a result, the main contact is closed to pass current to the motor2, thereby generating torque in the armature 10.

Then, the ECU 43 outputs a drive signal (turn-on signal) to the starterrelay 23. When the starter relay 23 is turned on, current is passed fromthe battery 6 to the solenoid coil 14 to operate the pinion-pushingsolenoid 5. With the operation of the pinion-pushing solenoid 5, thepinion movable body is pushed in the direction opposite to the motor viathe shift lever 4. Resultantly, the end face of the pinion gear 13 isbrought into contact with the end face of the ring gear 27. Then, at thepoint when both of the gears 13 and 27 have rotated to the positionsenabling engagement, the engagement between these gears is achieved.Thus, the torque of the motor 2 is transmitted from the pinion gear 13to the ring gear 27, whereby the engine EG is restarted.

In the starter 1 of the present embodiment, the solenoid coil 14 of thepinion-pushing solenoid 5 is formed of a single coil, and the solenoidcoil 14 is electrically separated from the motor circuit (i.e. thesolenoid coil 14 is not connected to the motor circuit). Therefore, thecircuit configuration can be simplified. In other words, some processes(e.g., a process of connecting one end of an attraction coil and one endof a holding coil to connectors or the like, and a process ofelectrically connecting the other end of the attraction coil to thefixed contacts 32 disposed on the motor side and configure the maincontact) can be eliminated. These processes would have otherwise beenrequired if the solenoid coil 14 is configured by two coils, anattraction coil and a holding coil.

In the starter 1 of the present embodiment, the field magnet 8 of themotor 2 is not required to be limited to a field electromagnet. Thus,either of a permanent magnet and a field coil may be usable. Use of afield coil will not necessitate establishing connection between thesolenoid coil 14 of the pinion-pushing solenoid 5 and field coil via anelectrical wiring.

In this way, the circuit configuration of the starter 1 can besimplified to thereby reduce the number of parts and the number ofmanufacturing processes. As a result, the starter 1 can be provided atlow coast.

Further, the starter 1 of the present embodiment enables independentoperation of the pinion-pushing solenoid 5 and the motor electrificationswitch 7. Therefore, when engine restart is requested during the enginestop process performed by an idle stop system, the engine EG can berestarted during the inert revolutions prior to the full stop. In thiscase, as described in the above item (b) explaining operation, the motorelectrification switch 7 is operated prior to the operation of thepinion-pushing solenoid 5. Specifically, current supply to the switchcoil 15 prior to the solenoid coil 14 will permit the motor 2 to rotateprior to the movement of the pinion movable body toward the ring gear27. Therefore, engagement between the pinion gear 13 and the ring gear27 can be achieved in the state where the relative numbers ofrevolutions of these gears in inert revolutions have been decreased.Thus, the engine EG startability can be enhanced, while the startingnoise can be reduced.

Furthermore, the pinion-pushing solenoid 5 and the motor electrificationswitch 7 are arranged in series in the axial direction AX. Hence,compared to a structure in which the solenoid and switch are arranged inthe circumferential direction CR, an area occupied when viewed in theaxial direction AX. In other words, an occupied size in the radialdirection RA of the motor 2 is kept smaller. Hence, the solenoid unitaccording to the present embodiment can be arranged in a mounting spacewhich is almost the same as a space required to mount a conventionaltype of starter electromagnetic switch with one plunger for both pushinga pinion gear and opening/closing the main contact.

Further, compared to a configuration in which the pinion-pushingsolenoid 5 and the motor electrification switch 7 are independent ofeach other in respect of their arrangement and structures, the solenoidunit of the present embodiment is still advantageous in that the numberof parts and manufacturing costs can be reduced. Unifying the cases ofthe solenoid 5 and switch 7 improves resistance to vibration applied.

The switch coil 15 is a single coil, so that, compared to the two-coiltype of switch coil, a winding step can be shortened in time and thecircuitry can be simplified. For the two-coil type of switch coil, twoterminal lines for grounding are necessary, while the one-coil type ofswitch coil needs only one ground-side terminal line. Hence, a step forprocessing the ground terminal line can be facilitated.

Second Embodiment

Referring now to FIG. 4, hereinafter is described an apparatus forstarting an on-vehicle engine according to a second embodiment of thepresent invention.

In the second and the subsequent embodiments as well as in themodifications provided below, the components identical with or similarto those in the first embodiment are given the same reference numeralsfor the sake of omitting explanation.

The second embodiment is associated with prolonging lives of thecontacts used in the starter relay 23 and the motor relay 53 describedin the first embodiment.

Since the configurations of the starter 1 and the solenoid unit (thepinion-pushing solenoid 5 and the motor electrification switch 7) arethe same as those in the first embodiment, the explanation is omitted.

The solenoid coil 14 of the first embodiment has not been formed of twocoils, an attraction coil and a holding coil. Instead, the solenoid coil14 of the first embodiment has been formed of a single coil in which oneend is connected to the starter relay 23 and the other end is grounded.Therefore, when the starter relay 23 is turned off and the solenoid coil14 is de-energized, a counter electromotive force (i.e., a surgevoltage) is generated by the inductance of the solenoid coil 14. Withthe generation of the counter electromotive force, current is passedthrough the starter relay 23. As a result, arc discharge occurs acrossthe contacts of the starter relay 23. Hence, the second embodiment isdirected to avoiding such arc discharges, while still gaining thevarious advantages described in the first embodiment.

With reference to FIG. 4, characteristics of a circuit configuration ofthe second embodiment, which differ from those in the first embodiment,are specifically described below. FIG. 4 is an electrical circuitdiagram illustrating the apparatus for starting an engine according tothe second embodiment.

In the pinion-pushing solenoid 5, a diode 46 is in parallel connected tothe solenoid coil 14. Likewise, in the motor electrification switch 7, adiode 47 is in parallel connected to the switch coil 15. In other words,in the pinion-pushing solenoid 5, the cathode of the diode 46 isconnected to the positive-potential side point, that is, the terminal22, of the solenoid coil 14 and the anode is connected to the groundingside. Likewise, in the motor electrification switch 7, the cathode ofthe diode 47 is connected to the positive-potential side point, that is,the terminal 34, of the switch coil 15 and the anode is connected to thegrounding side.

With the above configuration, when the starter relay 23 is turned off tode-energize the solenoid coil 14, the counter electromotive forcegenerated in the solenoid coil 14 can be absorbed by the diode 46.Specifically, the solenoid coil 14 is permitted to short-circuit by thediode 46 so that the counter electromotive force generated in thesolenoid coil 14 can be absorbed by the diode 46. Thus, since no currentpasses through the starter relay 23, arc discharge will not occur acrossthe contacts of the starter relay 23. As a result, wearing of thecontacts of the starter relay 23 can be suppressed, whereby the lives ofthe contacts can be suppressed from being shortened.

In the same way, when the motor relay 35 is turned off to de-energizethe switch coil 15, the counter electromotive force generated in theswitch coil 15 can be absorbed by the diode 47. Thus, since no currentpasses through the motor relay 35, arc discharge will not occur acrossthe contacts of the motor relay 35. As a result, wearing of the contactsof the motor relay 35 can be suppressed, whereby the lives of thecontacts can be suppressed from being shortened.

The two diodes 46 and 47 can be accommodated in a casing of the solenoidunit, which casing is formed of the overall case (the solenoid case 17and the switch case 18) and the contact cover 29. In this case, notbeing exposed to the outside, the diodes 46 and 47 can be prevented frombeing deteriorated. In addition, since the diodes 46 and 47 can beconnected within the casing of the solenoid unit, connector terminalsare not required to be newly provided.

In this way, in the second embodiment, the lives of the contacts used inthe starter relay 23 and the motor relay 35 can be prolonged. Theprolongation of the lives of the contacts is particularly effective in avehicle installing an idle stop system.

Specifically, the number of restarts of the engine EG is drasticallyincreased (e.g., by a factor of about ten) in a vehicle installing anidle stop system, compared to a vehicle not installing an idle stopsystem. Therefore, preventing wearing of contacts of the starter relay23 and the motor relay 35 for the prolongation of the lives of thecontacts is of extreme importance in the circumstances where use of idlestop systems is prevailing, and may also lead to enhancing reliabilityof the idle stop systems.

Third Embodiment

Referring to FIGS. 5 to 7, an apparatus for starting an on-vehicleengine according to a third embodiment of the present invention isdescribed.

The third embodiment is different from the first and second embodimentsin that a tapered projection 20 a is provided at the plunger 20 of thepinion-pushing solenoid 5.

FIG. 5 is a cross-sectional view illustrating a solenoid unit of thethird embodiment. As shown in FIG. 5, the plunger 20 of thepinion-pushing solenoid 5 is provided with the projection 20 a having atapered shaped. Specifically, the plunger 20 has an end face, in aradially inner side of which the tapered projection 20 a is providedbeing projected to and axially opposed to the core portion 16 b.Meanwhile, the core portion 16 b has an axial end face in which atapered recess 16 c is formed so that the projection 20 a of the plunger20 can be fitted thereto when the plunger 20 has been attracted to thecore portion 16 b.

Building up the plunger 20 by providing the tapered projection 20 a atthe end face may allow lots of magnetic flux to pass through theprojection 20 a. Therefore, compared to the electromagnetic switches ofthe conventional starters, the starter of the present embodiment canimprove saturation of the flux density to thereby increase theattraction force. FIG. 6 is a cross-sectional view illustrating anelectromagnetic switch used for a conventional starter. The“electromagnetic switches of the conventional starters” herein refers toan electromagnetic switch, as shown in FIG. 7, in which a singlemovement of the plunger 20 carries out both pushing a pinion movablebody and opening/closing a main contact, or refers to an electromagneticswitch not provided with the tapered projection 20 a at the end face ofthe plunger 20, which end face is opposed to the core portion 16 b (i.e.the plunger 20 with a flat end face).

FIG. 7 is a graph illustrating spring characteristics and attractionforce characteristics of an electromagnetic switch used for aconventional starter and the pinion-pushing solenoid of the presentinvention.

The electromagnetic switch of a conventional starter has a contactspring 41 (see FIG. 6) that pushes contacts, as well as the returnspring 25 and the drive spring 26. Therefore, as indicated by the brokenline (b) in FIG. 7, a required value of the attraction force becomeslarge at the time of achieving contact (at the time when the movablecontact 33 has contacted the fixed contacts 32). As a plunger gap (thevalue indicated on the horizontal axis in FIG. 7) becomes smaller, theinclination of the attraction force characteristics becomes drasticallylarge.

On the other hand, the pinion-pushing solenoid 5 of the presentinvention only has a function of pushing the pinion movable body towardthe ring gear 27, while the function of opening/closing the main contactis performed by the motor electrification switch 7. Therefore, therequired value of attraction force can be made small when the plungergap has a size corresponding to the size at the time of achievingcontact. In this regard, as indicated by the solid line (a) in FIG. 7,the attraction force can be increased in the present invention byproviding the tapered projection 20 a at the radially inner side of theend face of the plunger 20. The increase in the attraction force willlead to a decrease in the number of turns of the electromagnetic coil14, thereby making it possible to make the electromagnetic coil 14 morecompact in it size

In addition, the inclination of the attraction force characteristics canbe made small, whereby the attraction force characteristics may bepermitted to turn to the characteristics more suitable for the springcharacteristics.

As described above, the pinion-pushing solenoid 5 of the presentembodiment is provided with the projection 20 a at the radially innerside of the end face of the plunger 20. Therefore, the return spring 25can be arranged radially outside of the plunger 20 and the core portion16 b. Specifically, as shown in FIG. 5, one end of the return spring 25is held by a spring-holding recess 20 b formed in a radially outerportion of the plunger 20. The other end of the return spring 25 is heldby a spring-holding recess 16 d formed in a radially outer portion ofthe core portion 16 b. Thus, the return spring 25 is arranged close tothe inner periphery of the sleeve 24.

In this case, a lubricant, such as grease, may be applied to the innerperipheral surface of the sleeve 24, so that the plunger 20 can smoothlymove along the inner periphery of the sleeve 24. In this regard, withthe arrangement of the return spring 25 close to the inner periphery ofthe sleeve 24 as mentioned above, the lubricant dropped from the innerperipheral surface of the sleeve 24 can be temporarily collected betweenwire portions of the return spring 25. Then, when the plunger 20 hasbeen attracted to the core portion 16 b with the contraction of thereturn spring 25, the lubricant is pushed out from between the wireportions of the return spring 25 and returns to the inner periphery ofthe sleeve 24. Thus, lubricating properties can be maintained betweenthe sleeve 24 and the plunger 20.

Fourth Embodiment

Referring to FIG. 8, an apparatus for starting an on-vehicle engineaccording to a fourth embodiment of the present invention is described.

FIG. 8 is a cross-sectional view illustrating a solenoid unit of thefourth embodiment. In the fourth embodiment, the core portion 16 b isprovided with a projection 16 e. The projection 16 e has a tapered shapeand is formed so as to be axially opposed to the plunger 20 of thepinion-pushing solenoid 5.

More specifically, as shown in FIG. 8, the core portion 16 b has an endface, in a radially inner side of which the tapered projection 16 e isprovided being projected to and axially opposed to the plunger 20.Meanwhile, the plunger 20 has an axial end face in which a taperedrecess 20 c is formed so that the projection 16 e of the core portion 16b can be fitted thereto when the plunger 20 has been attracted to thecore portion 16 b.

Building up the core portion 16 b by providing the tapered projection 16e at the end face may allow lots of magnetic flux to pass through theprojection 16 e. Therefore, similar to the second embodiment andcompared to the electromagnetic switch of the conventional starter shownin FIG. 6, the starter of the present embodiment can improve saturationof the flux density to thereby increase the attraction force.

Similarly to that described in the second embodiment, the increase inthe attraction force will lead to a decrease in the number of turns ofthe electromagnetic coil 14, thereby making it possible to make theelectromagnetic coil 14 more compact in it size.

(Modifications) In the first embodiment, the pinion-pushing solenoid 5and the motor electrification switch 7 have been arranged in series inthe axial direction AX to integrally configure a solenoid unit.Alternatively, however, the solenoid 5 and the switch 7 may beseparately configured.

The diodes 46 and 47 of the second embodiment are not necessarilyaccommodated in the casing of the solenoid unit, but may be arrangedoutside the casing. The same applies to the case where the solenoid 5and the switch 7 are separately configured. For example, the diode 46may be arranged outside the casing of the solenoid 5, with the cathodebeing connected to the external connector terminal 22 and the anodebeing connected to the grounding side (e.g. to the solenoid case 17).Similarly, the diode 47 may be arranged outside the casing of the switch7, with the cathode being connected to the external connector terminal34 and the anode being connected to the grounding side (e.g. to theswitch case 18).

For the sake of completeness, it should be mentioned that the variousembodiments and modifications explained so far are not definitive listsof possible embodiments of the present invention. The expert willappreciate that it is possible to combine the various constructiondetails or to supplement or modify them by measures known from the priorart without departing from the basic inventive principle.

What is claimed is:
 1. A starter for a vehicle having an engine with aring gear, comprising: an electric motor that generates a torque inresponse to reception of electric power supplied from a battery via anelectric circuit electrically connecting the battery and the motor, thecircuit relaying the power; an output shaft that rotates in response toreception of the torque from the motor, the output shaft having alongitudinal direction defined as an axial direction; a movable memberhaving a pinion gear that transmits the torque to the ring gear andbeing movable on the output shaft together with the pinion gear in theaxial direction; a solenoid having an electromagnetic coil composed of asingle coil and electrically separated from the circuit, a fixed core,and a plunger, supply of excitation current to the electromagnetic coilallowing the fixed core to be magnetized to attract the plunger so thata movement of the plunger results in a push of the movable member towardthe ring gear in the axial direction; and a switch which is provided inthe circuit and which has a contact, a movable core, and a switch coilfunctioning as an electromagnet attracting the movable core in responseto supply of current to the switch coil, a movement of the movable coreresulting in on/off switching operations of the switch, the switch beingallowed to operate independently of the solenoid when both the switchand solenoid are controlled.
 2. The starter of claim 1, wherein theplunger has an end face opposed to the fixed core in the axialdirection, the end face of the plunger having a tapered projectionprojected toward the fixed core, the projection being tapered inward ina radial direction which is along a plane perpendicular to the axialdirection, the fixed core has an end face opposed to the plunder in theaxial direction, the end face of the fixed core having a recess whichallows the projection to be fit thereinto when the plunger is attractedby the fixed core.
 3. The starter of claim 1, wherein the fixed core hasan end face opposed to the plunger in the axial direction, the end faceof the fixed core having a tapered projection projected toward theplunger, the projection being tapered inward in a radial direction whichis along a plane perpendicular to the axial direction, the plunger hasan end face opposed to the fixed core in the axial direction, the endface of the plunger having a recess which allows the projection to befit thereinto when the plunger is attracted by the fixed core.
 4. Thestarter of claim 1, wherein the solenoid comprises a cylindrical sleevearranged along an inner circumferential surface of the electromagneticcoil and formed to slidably hold an outer circumferential surface of theplunger, and a return spring arranged between the plunger and the fixedcore such that, when the supply of the current to the electromagneticcoil is stopped, the return spring returns the plunger toward adirection from the fixed core, wherein the plunger has a recess formedon the outer circumferential surface thereof, the fixed core has arecess formed on an outer circumferential surface thereof, and thereturn spring has both ends one of which is held in the groove of theplunger and the other of which is held in the groove of the fixed core,both the recesses being stepped down from the outer circumferentialsurfaces of both the plunger and the fixed core.
 5. The starter of claim1, wherein the solenoid and the switch are arranged in series in theaxial direction so as to be unified as a solenoid unit.
 6. The starterof claim 5, wherein the solenoid unit comprises a solenoid case whichserves as both a case for the solenoid and a magnetic yoke for thesolenoid and a switch case which serves as both a case for the switchand a magnetic yoke for the switch, wherein the solenoid case and theswitch case being located in series in the axial direction and being asthe solenoid case.
 7. The starter of claim 1, wherein the switch coil isa single coil to which the current is supplied.
 8. An apparatus forstarting an engine mounted in a vehicle, comprising: a startercomprising: an electric motor that generates a torque in response toreception of electric power supplied from a battery via an electriccircuit electrically connecting the battery and the motor, the circuitrelaying the power; an output shaft that rotates in response toreception of the torque from the motor, the output shaft having alongitudinal direction defined as an axial direction; a movable memberhaving a pinion gear that transmits the torque to the ring gear andbeing movable on the output shaft together with the pinion gear in theaxial direction; a solenoid having an electromagnetic coil composed of asingle coil and electrically separated from the circuit, a fixed core,and a plunger, supply of excitation current to the electromagnetic coilallowing the fixed core to be magnetized to attract the plunger so thata movement of the plunger results in a push of the movable member towardthe ring gear in the axial direction; and a switch which is provided inthe circuit and which has a contact, a movable core, and a switch coilfunctioning as an electromagnet attracting the movable core in responseto supply of current to the switch coil, a movement of the movable coreresulting in on/off switching operations of the switch, the switch beingallowed to operate independently of the solenoid when both the switchand solenoid are controlled, an excitation circuit through which theexcitation current flows from an on-vehicle battery to theelectromagnetic coil; a starter relay that connects the battery and theexcitation circuit; a diode having a cathode and an anode, the cathodebeing electrically connected to a positive potential side point of theelectromagnetic coil and the anode being electrically connected to theground; and a controller that controls excitation and non-excitationoperations of the electromagnetic coil via the starter relay.
 9. Anapparatus for starting an engine mounted in a vehicle, comprising: astarter comprising: an electric motor that generates a torque inresponse to reception of electric power supplied from a battery via anelectric circuit electrically connecting the battery and the motor, thecircuit relaying the power; an output shaft that rotates in response toreception of the torque from the motor, the output shaft having alongitudinal direction defined as an axial direction; a movable memberhaving a pinion gear that transmits the torque to the ring gear andbeing movable on the output shaft together with the pinion gear in theaxial direction; a solenoid having an electromagnetic coil composed of asingle coil and electrically separated from the circuit, a fixed core,and a plunger, supply of excitation current to the electromagnetic coilallowing the fixed core to be magnetized to attract the plunger so thata movement of the plunger results in a push of the movable member towardthe ring gear in the axial direction; and a switch which is provided inthe circuit and which has a contact, a movable core, and a switch coilfunctioning as an electromagnet attracting the movable core in responseto supply of current to the switch coil, a movement of the movable coreresulting in on/off switching operations of the switch, the switch beingallowed to operate independently of the solenoid when both the switchand solenoid are controlled, an excitation circuit through which theexcitation current flows from an on-vehicle battery to the switch coil;a motor relay that connects the battery and the excitation circuit; adiode having a cathode and an anode, the cathode being electricallyconnected to a positive potential side point of the switch coil and theanode being electrically connected to the ground; and a controller thatcontrols excitation and non-excitation operations of the switch coil viathe motor relay.
 10. An apparatus for starting an engine mounted in avehicle, comprising: a starter comprising: an electric motor thatgenerates a torque in response to reception of electric power suppliedfrom a battery via an electric circuit electrically connecting thebattery and the motor, the circuit relaying the power; an output shaftthat rotates in response to reception of the torque from the motor, theoutput shaft having a longitudinal direction defined as an axialdirection; a movable member having a pinion gear that transmits thetorque to the ring gear and being movable on the output shaft togetherwith the pinion gear in the axial direction; a solenoid having anelectromagnetic coil composed of a single coil and electricallyseparated from the circuit, a fixed core, and a plunger, supply ofexcitation current to the electromagnetic coil allowing the fixed coreto be magnetized to attract the plunger so that a movement of theplunger results in a push of the movable member toward the ring gear inthe axial direction; and a switch which is provided in the circuit andwhich has a contact, a movable core, and a switch coil functioning as anelectromagnet attracting the movable core in response to supply ofcurrent to the switch coil, a movement of the movable core resulting inon/off switching operations of the switch, the switch being allowed tooperate independently of the solenoid when both the switch and solenoidare controlled, a first excitation circuit through which the excitationcurrent flows from an on-vehicle battery to the electromagnetic coil; astarter relay that connects the battery and the first excitationcircuit; a second excitation circuit through which the excitationcurrent flows from the on-vehicle battery to the switch coil; a motorrelay that connects the battery and the second excitation circuit; afirst diode having a cathode and an anode, the cathode beingelectrically connected to a positive potential side point of theelectromagnetic coil and the anode being electrically connected to theground; a second diode having a cathode and an anode, the cathode beingelectrically connected to a positive potential side point of the switchcoil and the anode being electrically connected to the ground; and acontroller that controls excitation and non-excitation operations of theelectromagnetic coil via the starter relay and excitation andnon-excitation operations of the switch coil via the motor relay. 11.The apparatus of claim 8, wherein the diode is incorporated in eitherthe solenoid or a solenoid unit formed by unifying the solenoid and theswitch in series in the axial direction.
 12. The apparatus of claim 9,wherein the diode is incorporated in either the switch or a solenoidunit formed by unifying the solenoid and the switch in series in theaxial direction.
 13. The apparatus of claim 10, wherein the first diodeis incorporated in either the solenoid or a solenoid unit formed byunifying the solenoid and the switch in series in the axial direction.14. The apparatus of claim 10, wherein the second diode is incorporatedin either the switch or a solenoid unit formed by unifying the solenoidand the switch in series in the axial direction.
 15. The apparatus ofclaim 8, wherein the apparatus is mounted in an idle stop apparatuswhich is capable of automatically controlling a stop and a restart ofthe engine, wherein the idle stop apparatus restarts the engine during aperiod of time a time instant at which the engine starts to stop to atime instant at which the engine stops completely, the engine rotatingduring the period of time due to inertia of the engine rotation.
 16. Theapparatus of claim 10, wherein the first diode is incorporated in eitherthe solenoid or a solenoid unit formed by unifying the solenoid and theswitch in series in the axial direction.
 17. The apparatus of claim 10,wherein the second diode is incorporated in either the switch or asolenoid unit formed by unifying the solenoid and the switch in seriesin the axial direction.
 18. The starter of claim 2, wherein the solenoidcomprises a cylindrical sleeve arranged along an inner circumferentialsurface of the electromagnetic coil and formed to slidably hold an outercircumferential surface of the plunger, and a return spring arrangedbetween the plunger and the fixed core such that, when the supply of thecurrent to the electromagnetic coil is stopped, the return springreturns the plunger toward a direction from the fixed core, wherein theplunger has a recess formed on the outer circumferential surfacethereof, the fixed core has a recess formed on an outer circumferentialsurface thereof, and the return spring has both ends one of which isheld in the groove of the plunger and the other of which is held in thegroove of the fixed core, both the recesses being stepped down from theouter circumferential surfaces of both the plunger and the fixed core.19. The starter of claim 3, wherein the solenoid comprises a cylindricalsleeve arranged along an inner circumferential surface of theelectromagnetic coil and formed to slidably hold an outercircumferential surface of the plunger, and a return spring arrangedbetween the plunger and the fixed core such that, when the supply of thecurrent to the electromagnetic coil is stopped, the return springreturns the plunger toward a direction from the fixed core, wherein theplunger has a recess formed on the outer circumferential surfacethereof, the fixed core has a recess formed on an outer circumferentialsurface thereof, and the return spring has both ends one of which isheld in the groove of the plunger and the other of which is held in thegroove of the fixed core, both the recesses being stepped down from theouter circumferential surfaces of both the plunger and the fixed core.